Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Flash memory cells should be erased prior to being programmed. A typical erase operation comprises grounding all of the word lines of the selected memory block to be erased. A positive erase pulse is then applied to the well of the selected memory block. The erase pulse is incremented until the selected memory block passes the erase verify operation.
For NAND flash memory, as the erase voltage increases, the cells on the edge rows of the memory block tend to degrade due to electron tunneling and trapping near the active diffusion region between the last gate stack in the selected memory block and the select gate. Over multiple erase cycles, this degradation can get worse, making it difficult for the edge cells to erase properly.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a way to erase non-volatile memory cells while reducing erase degradation.